Abstract

The coral cays of the Swain Reefs are far removed (up to 200 km) from any mainland influence. There is a complex inter-relationship between species distribution, species-turnover, and coral cay instability. Factors promoting cay instability include: (1) seasonal variation in wind direction, (2) short term cyclonic influences, and (3) longer term changes in the direction of the dominant winds. The cyclonic influence appears to be a major cause of inter-cay variations in plant species distribution. This is shown by comparing the orientation and position of the cays in 1964 with their present morphology. Gannet Cay for example only has a very small percentage of the vegetated area common to both the 1964 and 1984 cay outlines. A total of 13 species of terrestrial vascular plants have been recorded from the cays. Four species were prevalent and persistent since 1967. There was local extinction and immigration of plants which resulted in considerable species-turnover. Patterns of extinction and immigration varied among islands. In general, immigration was more prevalent early in the study and extinction more prevalent later. Unstable islands tended to have higher rates of species-turnover than did more stable ones.